1. Field of the Invention
The present invention is related to atomic force microscopy, which is a known technique for analysing the topography as well as the electrical characteristics of a sample, in particular a semiconductor device.
2. Description of the Related Technology
Atomic Force Microscopy (AFM) is recognised as an efficient technique for analysing very small semiconductor structures. AFM uses a probe, consisting of a very fine probe tip, which is mounted on a cantilever. The probe tip can be metal coated silicon, diamond coated silicon, full metal or full diamond. Diamond coated or diamond tips are used for analysing hard materials, such as silicon or SiO2. Silicon or metal tips are used for soft materials, such as InP, GaAs, or other III-V materials. During the measurement, the probe tip exerts a force on the sample, as it moves in a straight line over the sample under investigation. The movement is preferably performed by the tip with respect to a stationary sample. Subsequent scans of adjacent lines are performed in order to obtain a 2-dimensional analysis with a very high resolution, in the order of 10 to 30 nm in lateral direction. The movement of the probe tip in the direction perpendicular to the sample surface is detected, yielding an image of the sample""s topography.
In the case of a sample consisting of a semiconducting or conducting material, electrical characteristics, such as local resistivity and capacitance can be measured, which are used for example to calculate the doping level distribution in a semiconductor sample. These electrical AFM measurements are performed with the help of a polarisation voltage between the probe and a back contact of the sample, and a measurement of a resulting electrical value, for example a current, a voltage or a capacitance, provided that the tip is sufficiently conducting. Some of the documented techniques in this respect are:
xe2x80x98Scanning Capacitance Microscopy (SCM)xe2x80x99, and xe2x80x98Scanning Capacitance Spectroscopy (SCS)xe2x80x99, illustrated in the document xe2x80x98pn-junction delineation in Si devices using scanning capacitance spectroscopyxe2x80x99, H. Edwards et al, Journal of Applied Physics, vol. 87, no. 3, Feb. 1, 2000.
Tunnelling AFM (TUNA) and conductive AFM (C-AFM). These are techniques using current sensing for the electrical characterization of conductivity variations in highly-to-medium resistive samples. The use of AFM for measuring tunnelling currents is illustrated in the document xe2x80x9cNanoscale electrical characterization of thin oxides with conducting atomic force microscopyxe2x80x9d, A. Olbrich et al, proceedings of the 36th Annual International Reliability Physics Symposium, Reno, Nev., 1998, p. 163-168.
xe2x80x98Scanning Spreading Resistance Microscopy (SSRM)xe2x80x99, described in document U.S. Pat. No. 5,585,734, and related technique xe2x80x98Scanning Spreading Resistance Spectroscopyxe2x80x99. These techniques measure the resistance or conductivity of a semiconductor sample placed between the probe tip and a back contact, and derive from this the carrier profile of the semiconductor sample.
Nanopotentiometry, described in document U.S. Pat. No. 6,091,248. This technique measures an electrical potential in a semiconductor element with the help of an AFM microscope when applying/biasing one or more voltages over the semiconductor element.
Some of these techniques, in particular SSRM used on Si, require a high force between the probe tip and the sample, in order to obtain a good electrical contact. This high contact force quickly causes damage to the probe tip during scanning. To be more precise, the damage is mainly due to the lateral force (scratching), occurring while scanning in contact mode for high forces. This may lead to cleavage of sharp tips, or to a rapid increase of the tip radius of soft tips, the latter affecting the accuracy of the electrical measurements, as the contact radius is a determining parameter in the quantification. The larger the contact area between the probe tip and the surface under test, the smaller the resolution.
Other electrical measurements such as SCM and TUNA require smaller forces on the probe. The same is true for a good topography measurement. This dependency on the force level is the main problem when it comes to combining several measurements in one scan.
It has been shown that full metallic tips that are now available can be used to realize good electrical contact on silicon when remaining static, i.e. in point contact under a high contact force, whereas in scanning mode, the metal probe tip degrades almost immediately. One can release the probe tip and move to another point to avoid scratching of the sample, but no information will be obtained about the points lying in between the two measurement points. Because the full metallic and metallic coated Si-tips are unable to withstand high lateral forces, diamond coated Si-tips and full diamond tips have been introduced. Limitations of all diamond-containing tips are their price in terms of material as well as production cost, and the fact that diamond can only be doped to a limited concentration level of 3 ppm. Therefore, the resistance of the diamond probe remains relatively high (1 to 10 kxcexa9) and it is not possible to measure highly doped areas (above 1020 atm/cm3). One approach, wherein the force between the probe and the sample varies is the so called tapping mode technique, see document U.S. Pat. No. 5,412,980. In this technique, the probe is oscillated in a direction perpendicular to the sample at a frequency close to its resonant frequency, touching the sample only periodically. This technique is useful for a topography measurement, but is unsuitable for the measurement of electrical properties.
One aspect of the present invention provides a method and apparatus which allow the gathering of electrical data of various kinds while simultaneously measuring the topography of the sample. Another aspect of the invention is that such a method and apparatus should maximize the life of the probe tip and reduce the cost of operation.
Another aspect of the present invention provides a method for performing an atomic force microscopy (AFM) measurement, using an AFM microscope equipped with a probe, the method comprising: defining a force profile, which is a force change as a function of time, wherein the minimum force is larger than zero; and performing the AFM measurement on a sample, by scanning at least one line over the sample, wherein a force being exerted on the sample by the AFM probe is changed according to the force profile, during the movement of the probe in one direction along the line.
In another aspect of the invention, the method may further comprise defining a speed profile, which is a speed change of the probe as a function of time, and wherein the speed of said probe during the movement along said line changes according to said speed profile. In the method, said force profile is a periodic block wave, each cycle of said block wave consisting of an interval wherein said force is constant at a first level, followed by an interval wherein said force is constant at a second level, and wherein said first level is higher than said second level. In the method, said speed profile may equally be a periodic block wave, wherein said speed is constant during the intervals of said second force level, and wherein said speed is zero during the intervals of said first force level. Alternatively, said speed profile may be constant. In the method, electrical data as well as topographical data are gathered during one movement of said probe along one line. When the force profile is a block wave, at least one electrical measurement is performed during said first interval and wherein at least one topographical measurement is performed during said second interval.
Yet another aspect of the invention provides an apparatus for performing an atomic force microscopy measurement, comprising: an Atomic Force Microscope (AFM); an amplifier; an input/output device; and a controller device; wherein the input/output device is configured to receive at least one of a force profile and a speed profile, and to transfer the at least one of the profiles to the controller device, and wherein the controller device is configured to transmit the at least one of the profiles to the AFM, and wherein the apparatus is configured to perform a multiplexing/demultiplexing operation on the measurement data coming from the AFM, and wherein the amplifier amplifies the measured signals before performing the multiplexing/demultiplexing operation and transmits the measured signals to the controller device.
Still another aspect of the invention provides an apparatus for performing an atomic force microscopy measurement, comprising: an atomic force microscope (AFM) having a probe with a tip, the AFM configured to perform an atomic force microscopy measurement on a sample; and a controller configured to receive a force profile which is a force change as a function of time, and to control the AFM such that a force being exerted on the sample by the probe tip is changed based on the force profile during scan of at least one line on the sample.
Still another aspect of the invention provides a method of performing an atomic force microscopy measurement, comprising: Providing an atomic force microscope (AFM) including a probe with a tip; defining a force profile, which is a force change as a function of time; scanning at least one straight line over a sample using the prove tip; and changing a force being exerted on the sample by the probe tip based on the force profile during the scanning.